1. Field of the Invention
The present invention relates to an optical transmission system and an optical transmission method using an optical signal capable of transmitting information of a plurality of bits within one code (one symbol time). More specifically, the present invention relates to a technique for realizing excellent transmission characteristics by compensating or correcting an error in a received signal caused by an optical signal modulation format or a multiplex system.
2. Description of the Related Art
With recent popularization of the Internet, there is an increasing demand for a basic optical communication system having a transmission capacity equal to or higher than 40 Gbit/sec. As a method for realizing this, adoption of various optical modulation formats having excellent spectral efficiency, Optical Signal-to-Noise Ratio (OSNR) resistance, and nonlinearity resistance, as compared with a Non Return to Zero (NRZ) modulation format, which has been applied to a conventional optical communication system with a transmission capacity of 10 Gbit/sec or less, has been sought. Against this background, a system that transmits multi-bit information within one symbol time has been attracting attention. For example, 40-Gbit/sec and 100-Gbit/sec optical transmission systems, which combine Quadrature Phase Shift Keying (QPSK) and polarization multiplexing, have been discussed actively (for example, refer to C. Laperle et al., “Wavelength Division Multiplexing (WDM) and Polarization Mode Dispersion (PMD) Performance of a Coherent 40 Gbit/s Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) Transceiver”, OFC '07, PDP16, 2007, and H. Masuda et al., “20.4-Tb/s (204×111 Gb/s) Transmission over 240 km using Bandwidth-Maximized Hybrid Raman/EDFAs”, OFC '07, PDP20, 2007).
In the optical transmission system using a multivalue modulation format, a polarization multiplexing transmission system, or a digital coherent receiving system, logic of a transmitted signal may be inversed bit by bit at the time of reception, due to factors such as a change in an operating point of an optical transmitter, an initial phase of a local oscillator light, and a phase fluctuation in a signal light and the local oscillator light. Moreover, in addition to the above factors, due to alternation of polarization at the time of reception, a phenomenon referred to as bit swap, in which an order of received bits is alternated, may occur.
For example, in the optical transmission system adopting the digital coherent receiving system combining QPSK and polarization multiplexing as shown in FIG. 17, there can be considered a possibility that logic inversion and bit swap may occur dynamically in a pattern as shown in FIG. 18. More specifically, in the optical transmission system, lights generated by a light source 1102 in an optical transmitter 1100 are divided into polarization components orthogonal to each other by a polarization beam splitter 1103, and respective lights of X polarization and Y polarization are further branched into two, and respectively provided to two phase modulators 1104. The lights input to each phase modulator 1104 are phase-modulated according to a transmission signal processed in a transmission signal processing circuit 1101. After one phase of a pair of phases corresponding to the polarized waves is shifted by π/2 by a phase shifter 1105, respective lights are synthesized by a polarization beam combiner 1106. Accordingly, a 4-bit coded optical signal is transmitted from the optical transmitter 1100 to an optical receiver 1300 via a transmission line 1200. In the optical receiver 1300, the optical signal from the transmission line 1200 and a local oscillator light output from a local oscillator light source 1301 are provided to a polarization diversity 90° hybrid circuit 1302, and the output lights of the circuit 1302 are converted into electric signals by photo detectors 1303. After the received signals are AD-converted by AD converters 1304, the signals are provided to a digital signal processing circuit 1305 and a received signal processing circuit 1306, where 4-bit code demodulation processing is performed. Accordingly, 4-bit (16-valued) information is transmitted between the optical transmitter 1100 and the optical receiver 1300 within one symbol time. In FIG. 17, 4-bit transmitted data encoded by the optical transmitter 1100 are designated as A, B, C, and D, and 4-bit received data demodulated by the optical receiver 1300 are designated as A′, B′, C′, and D′.
In such an optical transmission system, logic inversion in 16 patterns as shown in the upper part of FIG. 18 may occur dynamically, due to; a bias point of the phase modulators 1104 in the optical transmitter 1100, an optical path difference between the polarization beam splitter 1103 and the polarization beam combiner 1106, polarization mode dispersion (PMD) in the transmission line 1200, nonlinear phase noise, an optical path difference between polarized waves in the optical receiver 1300, or a phase fluctuation in the local oscillator light source 1301. Moreover, bit swap in 8 patterns as shown in the lower part of FIG. 18 may occur dynamically, due to; the phase fluctuation in the local oscillator light source 1301, alternation of polarization channels (X polarization, Y polarization) at the time of reception, or the nonlinear phase noise.
Although not shown specifically, in the optical transmission system adopting the digital coherent receiving system or a direct detection system, in which Differential Quadrature Phase Shift Keying (DQPSK) and polarization multiplexing are combined, dynamic logic inversion is not caused by performing differential reception, and hence, the number of logic inversion patterns decreases. Moreover, the bit swap in eight patterns as shown in the lower part of FIG. 18 may occur, though not dynamically, due to a bias of the phase modulator on the transmission side, or alternation of the polarization channels.
In order to avoid a bit error in the received data with respect to the logic inversion and bit swap, high-speed logic inversion control and multiplexing timing (bit swap) control need to be performed in the optical receiver. As one example of the conventional technique involved with the control, a method for controlling the logic inversion and bit swap by using frame synchronization detection is known for the DQPSK system using direct detection (for example, refer to Japanese Unexamined Patent Publication No. 2006-270909).
The conventional control technique using frame synchronization detection is effective for controlling logic inversion and bit swap occurring when 2-bit (4-valued) information is transmitted within one symbol time. However, it cannot correspond to the logic inversion and bit swap occurring when information larger than 2 bits is transmitted within one symbol time as in the optical transmission system combining Quadrature Phase Shift Keying (QPSK) and polarization multiplexing. Moreover, because it is a method of detecting the logic inversion based on whether there is frame synchronization in the directly detected received signal, there is a problem in that application to the optical transmission system using the coherent receiving system is difficult.
Moreover, when multi-bit information is transmitted within one symbol time by using the multivalue modulation format or the like, regardless of whether the conventional control technique using the frame synchronization detection is applied or not, a characteristic variation may occur between transmission channels. Particularly, when it is attempted to expand the transmission capacity by combining the polarization multiplexing system, there is concern that the characteristic variation between the X polarization channel and the Y polarization channel may increase. With such a characteristic variation between the channels, there is a problem that this may cause deterioration of error correction performance when error correction is performed for the received signal by using, for example, a known error correction code. In other words, since error correction using the error correction code is a technique assuming random error, the characteristic variation between the channels deteriorates the random nature of the error in the received signal, thereby causing deterioration of the error correction performance.
FIG. 19 shows one example of a bit error rate (BER) characteristic before and after the error correction according to the presence of a characteristic variation between channels. The X axis denotes a mean value of the BER (Input BER) for all channels before the error correction, and the Y axis denotes a mean value of the BER (Output BER) for all channels after the error correction. From FIG. 19 it is seen that when the random error does not occur due to the characteristic variation between respective channels of the received signal, deterioration of an improvement amount of the Output BER with respect to the Input BER, that is, of a coding gain, occurs.
Here, causes which generate characteristic variation between the channels will be described in detail.
For example, the most common cause of characteristic variation between the channels in the polarization multiplexing system is a polarization dependent loss (PDL) in a transmission line and an optical device. In the optical transmission system as shown in the upper part of FIG. 20, when there is a PDL as shown in the middle part of FIG. 20, the power of the optical signal transmitted to a transmission line 2002 is controlled to be constant in the output of an optical amplifier 2001 in each repeater span. However, due to the PDL in the transmission line 2002, a power difference occurs between the X polarization channel and the Y polarization channel, and hence, although the average power (Ave) of the X polarization channel and the Y polarization channel becomes constant in each repeater span, the power difference between the X polarization channel and the Y polarization channel gradually increases with an increase of the number of repeater spans. Therefore, as shown in the lower part of FIG. 20, a difference in the OSNR between the X polarization channel and the Y polarization channel gradually increases, and as a result, a characteristic variation occurs between the different polarization channels.